Sunday, May 23, 2010

87> PRT Track and Bridge Design

I have been trying to devise the absolute best PRT track design I can come up with, with the help of you, the readers. My goal is to offer an alternative transportation infrastructure that beats the pants off of roads by every metric, save for (initially) the network effect. In my opinion, only then will politicians and transit decision-makers have the political cover to move forward with a PRT program.

The search for the best track design brings up some inevitable questions, such as “How far apart can the supports be before the stability or economy of the system is compromised?” and “How high should the track be?”

The first question leads to an examination of bridge design. Trusses with minimal girth, such as Pratt, Howe or Warren trusses) seem to top out at about 100 feet. The most economical way to span long distances, however, lies in the introduction of tension elements, such as is seen in a suspension or cable-stayed bridge. The most extreme case of a tension maximizing (compression minimizing) design would be the cable/tower arrangements that support ski-lift gondolas. Some have proposed such means as an urban transit system, and they certainly would have something there were it not for the need for multiple origins and destinations.

 It would be an easy matter to design a flexible track and simply pull it tight, though sagging in the middle would be both unavoidable and unacceptable. Pulling this sag up with cables would seem to be the cheapest way to span long distances with a minimum profile track. 

Suspension Bridges can traverse the longest distances, but cable-stayed bridges (above) offer other advantages. A good explanation of how the two compare is found about halfway down this Wikipedia page. ttp://  The gist of  it is that suspension bridges have vertical cables from which the load bearing structure hangs, while cable-stayed bridges’ hanging cables are angled from a support column so that the load bearing structure is compressed by its weight.

Many of you may have seen the late Hans Kylberg’s dramatic use of cable-stayed PRT track in the Bubbles and Beams video or in this illustration. Below is a picture of a curved bridge section supported by cables.

The use of cables does present a question of aesthetics. Whereas I think most people would find both types of cable supported bridges generally attractive, there is a question of too much of a good thing. Instead of a minimalist design, there would be support columns and cables everywhere. What looks good crossing a river might not look so good close-up on your street. I have given some thought to reducing the tower height and I will post my ideas for that in the near future.  

Then there is the question of track height. One obvious way to mollify the NIMFYs (Not I My Front Yard) is to have the system so elevated that it is unobtrusive. An advantage to the system I have been advocating is that it can easily and steeply climb or descend to any desired level. There is the matter of emergency evacuation, but I think that is manageable. So how high is too high? Should this be a system that can whisk you along above the trees? That would be my preference, but everyone has his own sensibilities. What are yours?


qt said...

How high is too high? Well, this might be a place to start thinking:

The blog itself is interesting, by the way--if only as another point of view.

Ryan Baker said...

Would this be cheaper than the standard truss and column approach? I'm thinking no, but have no hard data.

My reaction is that raising the visual impact would probably be harmful in most cases. The downtown concept is fairly obviously less attractive since you would now have cables going past maybe dozens of floors worth of windows.

In less dense areas the cables would be less of a problem, but my gut still says that preferences would go for staying away from the cable spans.

As a spot element of a system I think this works well, and if done well could be aesthetically pleasing at times.

cmfseattle said...

seems to me, much of the auto's success lies in the clouding of true costs. if we really account for all "externalities" (ahem, top kill, cough), do the benefits honestly outweigh the costs?

there are some tasks (e.g., building construction) that would rarely be practical via PRT, so robocars are probably worth developing. but power and comms utilities can be mounted in guideways, and neighborhood-scale energy storage systems would be a good match for sustainable generation methods.

the costs of arterial guideways should be weighed against all of their potential.


a few years ago, i thought a suspended system would be better because the floor of a vehicle could be turned into an evacuation method: an emergency lever would be thrown, which unlocked a geared wheel. four chains with movable netting around the perimeter would allow the passengers to lower themselves the ~20 feet in <30 seconds.

Dan said...

Dan The Blogger appreciates all comments but is about to be kicked out of the restaurant! Be back soon.

Dan said...

Dan the Blogger is back, sort of…

Qt: I haven’t checked it out in detail yet, but it does seem interesting…The moving cable idea has always intrigued me. I saw one proposal where magnets were conveyed under a roadway to pull vehicles along, or was it just steel, and the magnets were in the vehicle…I certainly have, and still am, considering such a scheme for steep slopes.

Hi Ryan: First, let me thank you for adding your input to some of the older threads. I encourage others to do the same! In theory, it costs less to remove solid truss work in lieu of inexpensive cabling. In practice…well, that remains to be seen. I have some ideas that will be forthcoming. I should point out, though, that the cabling need not be the kind of extensive double fan/tall tower type of system that is usually seen. I would think even a single cable would extend the span at least somewhat.

Cmfseattle: You are sure right about the hidden costs. It’s hard to argue against a system whose many problems are both so well hidden and yet so familiar. I will hold off on the robocar thing.

What are the cons of your evacuation method, other than it would be pretty tough if you were stuck over a highway?

Ryan Baker said...

I first got interested in PRT through the robotic car route. I knew what was going on with the DARPA Grand Challenge and could see this wasn't that far off, so I started thinking about the different ways you could take advantage of that capability.

Those thoughts led me to a concept of separating the cabin and drive system (I called them sleds) so that you people could keep their personal cabins, but they could be stored in about a fifth of the space. In addition you could share the sleds and have a more stringent safety regimen without personal ownership of the navigation/suspension/propulsion.

As you can imagine those thoughts eventually led me to become aware of the dual mode PRT systems, and my thoughts evolved into considering how the cabins could also be loaded onto a PRT sled so that the benefits of inexpensive elevated track (efficiency and no interference) could be imparted to shared with the robocar system.

It took a while, but it was one of Dr. Anderson's papers that finally convinced me that dual mode wasn't the most near term aspects of the system.

I still think my initial concepts have long term promise and utility, but I'm kind of hoping single mode PRT gets a good headstart of robocar development. I'm a little afraid that if robocar's make themselves available before SM takes off, we may see the transportation infrastructure stick with big concrete roads, and make a lot less progress in densifying our infrastructure (i.e., walkable neighborhoods, less sprawl).

See, if robocars would give users all the user-experience advantages PRT promises, but would miss some efficiency and system advantages. Sure you could add those later, but it would sure be a lot harder once the candy has already been handed out.

qt said...

The cable setup is indeed intriguing, for certain situations especially. But of course the particular post seemed especially relevant to this discussion.

Little things like backyard privacy often don't show up in discussions like these. Kind of like the "car as personal statement" aspect of transport you mention in your latest post. And if someone does mention such a thing, as often as not it's to tell those people they need to get over it. Not a good way to drum up support.

A later post on that gondola blog, by the way, spoke of an interesting unintended consequence of the gondola system in Caracas (I believe). Street crime along the route dropped precipitously, he says. Something about a gondola full of eyewitnesses with cell phones coming overhead every ten seconds or so seemed to discourage the muggers...

Andrew F said...

I imagine the backyard privacy issue to be somewhat muted with PRT, as I imagine the most typical application would use existing public right of ways (ie, roads). I find it hard to imagine them passing over homes.

A perhaps bigger problem is that you'd have a stream of gawkers passing second or third floor windows of houses and apartment blocks. One idea, of which I'm uncertain of the practicality, would be to use complementary polarizing film on podcar windows and the windows of any buildings adjacent to the guideway. They would work such that anything viewed through just one of these films would be visible, but anything viewed through both would not. Again, I'm generally ignorant of how optics works, but I saw something about how the latest generation of 3D glasses work, and that caused me to think about this problem.

Dan said...

Ryan – I hear what you are saying. Indeed, the limitations of track are kind of an asset when it comes to city planning. I personally like the hanging style PRT because it is sort of like a personal helicopter for the user in that it can get into just about anywhere without worry about crossing tracks or only using elevated stations. It is a very versatile medium for the system/route designer. The slopes might have to be steep or some corners tight, but it can find a way there, and it will be cheap.
Later, when we are used to the beauty of thin beam rail travel, we can adapt our cars to run on, (or hang from) them. It will seem like the logical thing to do. Right now the logical thing to do, when building a raised guideway, (ULTra style) is to go the extra mile and just make it bigger…just in case. Then we just end up with more of the same.
Qt – That is interesting indeed about the drop in crime…I had originally thought that perhaps the windows in a PRT vehicle might be so situated so as to make the passenger have to sit up and forward to get a good side view. This is a very important topic that needs to be addressed. I can’t thank you enough for putting it front and center.

Andrew – I am very familiar with the concept, so I will explain it to the readers. Polarized lenses, such as those used in sunglasses, block all light waves except for those with a specific orientation, say, for example, vertical. If a second such lens is placed in front of the first, oriented in the same way, you can see through them both. However if you turn the second lens so it is oriented sideways to the first (horizontal) then the two together will not allow the passage of any light. Try it next time you are at a drugstore sunglasses display. Turn two pair at angles to each other and voila, opaque. Twist one pair, and voila! You can see through both pair. Magic. Interestingly, if you place most natural crystals between the crossed polarized lenses they will allow light to pass. (Doesn’t work with diamond, topaz or other cubically crystallized minerals) So there you have it. Is it amethyst or purple glass? You can tell with a couple pair of sunglasses!

Sorry, I’m a science guy… Interesting application, Andrew. I suspect if any homeowners would want to put polarizing film on their windows there is already a very big problem however. (Like backyard privacy) This sounds like an interesting application for “smart glass” which can be made opaque with electric current. All vehicles could have the side window fog up as they pass the shy homeowners and clear up as soon as they pass. It’s a laminated liquid crystal technology that might be practical if the side windows are designed flat, since that is how it comes. Oops. There goes the crime rate…

Andrew F said...

I was thinking of that as well. Maybe OLED would be practical for that in the future, since it can be applied as a flexible film. I'm not sure it could block light, but it should be at least able to frost the windows on the vehicle. The reason I didn't suggest that is that it actually seemed more cumbersome than polarizing film, because the former is active while the latter is passive. Of course it requires applications to every window instead of just the vehicle, but only those homeowners concerned with the gawking (such as it is at 60 kph) would likely go through the expense and bother. Also, it means that the homeowner and the PRT occupant can still see everything else outside, just not each other.

Ryan Baker said...

One advantage to OLED would be that the PRT passengers could have privacy too if they wanted it. Tap one button and the windows will be frosted even when in open space.

Still I like Andrew's idea better. Like he said, then the PRT passengers wouldn't have to live with a window that keeps flashing opaque to transparent, plus I'm thinking it's a bit cheaper, at least for the vehicle.

I also kind of have an inclination that it may generally be an unnecessary concern. People in the city are used to using blinds and such for these purposes. As long as you have consistent orientation of the polarization in the glass, the home owners can apply a film or replace their windows if they want.. or they can use blinds.

qt said...

Glad to have caused some more discussion. Two little points.

The technical solutions to the "looking into the second story" problem are interesting and potentially useful. But of course my original comment was meant as one possible answer to the question Dan the Blogger asked in that last paragraph. "How high is too high?" The answer, naturally, is "It depends." Whisking us along above the treetops is a good thing sometimes. Other times it's in intrusion--on the privacy of the people we pass, on the aesthetics of the neighborhood, or on some quality we haven't thought about yet.

One of the nice things about "open source PRT" might well be that you can do a LOT of variations, to fit very specific needs and/or objections brought up in very specific local situations--IF you pay attention to the people you're supposed to be serving.

A lot of the solutions that have shown up here have involved things like polarized films to be applied to people's windows, etc. Nice technical coups that look real good on paper. Have you thought of how the people in those neighborhoods will react to "Well, if you have a problem with this you can always spend money to take care of it. Or maybe we'll pay for the film. You'll get X percent less light through your front windows, but that's a small price to pay for the wonderful world of tomorrow, right?"

As I said in my second comment here, it's real easy to be so in love with something like PRT (or dualmode, or light rail, or gondolas...) that your answer to any objection is a quick tech fix or to toss off something like "You'll get used to it." or "You'll just have to adapt." The people objecting might not have quite the same view of things...

"Politics," if I've got it right, is derived from the Greek word for "city," and refers to the skills needed to keep a city running and its people away from each other's throats. "Civility" is derived from the Latin word for "city," and means "the minimum level of good manners needed to allow people to live this close together." Politics and civility are frequently overlooked by reformers, whether they're trying to save the country from the wrath of God, or bringing about a utopian civic brotherhood, or building the perfect transportation system. Something to keep in mind.

Andrew F said...

Great points, qt. I certainly don't disagree that you can't just throw technical fixes at every problem, and it is often better to address the root cause than the to treat the symptom.

I think this ought to be a fairly minor problem. I imagine most PRT running on busier main streets, where there is little privacy at present. There will be times where PRT will run through residential areas, and we need to have some solutions that can address concerns of residents, who are unlikely to support a PRT line running past their home.

I imagined the biggest problem with condos and apartments that are set near the street, with full floor-to-ceiling windows. I think houses, set some distance from the street, would be less of a concern. Of course, running a PRT line above someone's property is bound to raise considerable ire. Seems that this is the problem with the gondola system you linked to.